In many nuclear experiments, the energy efficiency of detector is a param eter without negligibility.. In this paper, the absolute efficiency of HPGe detector is surveyed and mearsured a
Trang 1V N U J O U R N A L O F S C IE N C E , M a th e m a tic s - P h y s ic s , T.xx, N 02, 2 0 0 4
S U R V E Y I N G T H E H P G e G A M M A
D E T E C T O R A B S O L U T E E F F I C I E N C Y
T r a n T ri V ie n , D o a n Q u a n g T u y e n , T r a n V i e t N h a n H a o ,
D o a n T h a n h S o n , N g u y e n T r u n g T i n h
College o f Science, V N U
A b s tra c t In many nuclear experiments, the energy efficiency of detector is a
param eter without negligibility In this paper, the absolute efficiency of HPGe
detector is surveyed and mearsured at different distances from detector and
different gamma energies
1 I n t r o d u c t i o n
I n m an y n u c le a r e x p e r im e n ta l m e a s u r e m e n ts , t h e d e te r m in e d re s u lts
d e p e n d on e x p e r im e n ta l p a r a m e t e r s , one of th e s e p a r a m e t e r s is a b so lu te detector efficiency B u t u n f o r t u n a t e l y th e efficiency of n u c le a r d e te c to r is n o t co n stan t, it
d e p e n d on th e e n erg y of m e a s u r e m e n ta l ra d ia tio n So t h a t , m ak in g efficiency
c a lib ra tio n of d e te c to r is n e c e ssa ry I n th is p a p e r, t h e a b s o lu te efficiency c alib ratio n
of H P G e g a m m a d e te c to r is su rvey ed
A b s o l u t e e f f i c i e n c y o f d e t e c t o r TỊabs a t a e n e r g y v a l u e i s d e t e r m i n e d t h r o u g h ,
p h o to p e a k a re a , s , by e q u a tio n below:
w here: A -activity of ra d io a c tiv ity source
I - ra d io a c tiv ity e m iss io n p ro b ab ility
t - m e a s u r e m e n ta l tim e
As e q u a tio n (1), t h e e rr o r of efficiency d e p e n d on t h e p h o to p e a k a rea (S) In
o rd er to d e c re a se s tro n g ly th e e rro r of efficiency T h e m e a s u r e m e n ta l a re a of
p h o to p ea k sh o u ld d e te r m in e w ith h ig h precision To reso lv e t h is problem, some
d e te r m in a tio n p h o to p e a k a r e a stu d ie d :
- T h e to ta l p e a k a r e a a p p ro x im a tio n
- T h e Covell m e th o d
- T h e W a sso n m e th o d
Beside, th e s u p e rp o sitio n , d e a d tim e, t h e effect a r e co nsided
In th is p a p e r , th e g a m m a H P G e d etecto r efficiency is d e te r m in e d th ro u g h
th e p h o to p e a k a r e a of s t a n d a r d sources, th e d e te r m i n a ti o n of d e te cto r efficiency is able to be p e rfo rm e d by c a lc u la tio n B u t w ith t h is m e th o d , th e d etecto r geometry
44
Trang 2Surveying the HPGe g a m m a d e t e c t o r a b so l u te efficiency 45
h as to be known So, in e x p e rim e n t, th e efficiency c a lib r a tio n m e th o d is com bined
w ith se m i-em p irial r e la tio n be come m o st reliab le
In order to re je c t t h e in flu en ce of th e d is to r tio n of p h o to p e a k s h a p e d ue to th e high activity a n d by th e c o u n tin g loss d ue to th e pile-up effects, th e sa m p le sh o u ld
be p u t at place w ith d ifferen t d ista n c e s to detector
For fittin g th e e x p e r im e n ta l d etecto r efficiency d a t a w i t h th e o ric a l fuction In
th is paper, two th e o ric a l fu n ctio n s d escrib in g th e d e p e n d e n c e of efficiency on energy are used, such as:
r| = y H.lnte) 1
1 = 1 ( 1 , ^ '
2 E x p e r i m e n t a l a b s o l u t e e f f i c i e n c y c a l i b r a t i o n
For g e ttin g e x p e r im e n ta l efficiency v a lu e T h e so u rc es w ith d ifferen t g a m m a ray energies a re u s e d in o u r e x p e rim e n t: E u 152, C s 137 a n d A m 241 T h e so u rces w ith
sh ap of disk w ith 1 Cm r a d i u s a re su p p lie d by IAEA w ith p a r a m e t e r s as following:
S o u r c e : E u 152
Half-life: 12.7 Y ear
D a te of produce: A u g u s t l 8t 2002
A ctiv ity in itia l: 3672.62 Bq
S o u r c e : A m 241
Half-life: 433 Y ear
D a te of produce: J u l y 15th 2002
A ctivity in itial: 3759.2 Bq
S o u r c e : C s 137
lf-life: 30.1 Y ear
D a te of produce: D ecem ber l 8t 1994
A ctiv ity in itia l: 36445 Bq For efficiency c a lib r a tio n of d etecto r, in o u r e x p e r im e n t we h a v e to know th e activity of source a t th e e x p e r im e n ta l tim e T h is w o rk is n o t difficult by u sin g equation:
A=A0e Xt
T he p a r a m e t e r s of e x p e r im e n t w ith H P G e g a m m a v is io n s p e c tro m e try are given in tab le 1 a n d plot of e x p e r im e n ta l efficiency c a lib r a tio n i n fig l
Trang 346 T r a n Tri Vien, D o a n Q u a n g Tuyen, T r a n Viet N h a n Hao
T a b l e l The efficiency of detector depend on energies a t 2 cm from detector
Order
Number
Energy (KeV) (%)It
Count Count pel'
second
Activity
of source
Detection efficiency
After fittin g th e e x p eim e n tal d a ta with th e theorical fun ctio n :
’1 =
th e absolute efficiency function of H PG e gam m avision sp e c tro m e try is as following:
11, (E) = 11.5436 E '1- 1110.34 E '2 + 60344.9 E 3 -2008730 E 1 rj2(B) =-3.09414(lnE) 1+73.7661(lnE )'2-68 7 0 9 8 (ln E )'3+ 2 9 6 7 1 l( ln E ) 4-4814.21(lnE)'5
(3)
Absolute
efficiency
0.04
0 0 3
0.02
0 01
Fig 1 The dependence of efficiency of detector on e n erg ies a t 2 cm from detector
Trang 4Surveying the HPGc g a m m a dete cto r absolute efficiency 47
T a b le 2 The efficiency of detector depend on energies a t 5 cm from detector Order
Number
Energy (KeV) (%)Iy
Count Count per
second
Activity
of source
Detection efficiency
A b s o l u t e
e f f i c i e n c y
Fig.2 The d e p en d en ce of detector efficiency on th e energies a t 5cm from detector
After fiting th e e x p e im e n ta l d a ta with th e theorical function:
Trang 548 T r a n T ri Vien, D o a n Q u a n g Tuy en, T r a n Viet N h a n Hao
th e a b so lu te efficiency fu n c tio n of H P G e g a m m a v isio n sp e c tr o m e tr y is as following:
6l(E) = 1.65095 E 1 - 30.281 E '2 - 3103.08 E 3 e2(E) = 0 3 6 0 4 5 8 ( ln E ) 1-7.62022 (InE)-2 + 4 3 9 9 1 6 (ln E )'3 -8 4 6 5 7 1 (ln E )4 (4)
F i g 3 T h e d e p e n d e n c e of a b s o lu te efficiency of d e te cto r on e n e rg ie s of HPG e gam m a
G a m m a v is io n sp e ctro m etry
Absolute
efficienc
Fig.4 The d e p e n d e n c e of a b so lu te efficiency of d e te cto r o n e n e rg ie s of HPG e gam m a
G e n n ie 2000 s p e c tro m e try
Trang 6Su rv eyi ng the HPGc g a m m a d e te c t o r a bs o lu te efficiency 49
In order to d e te r m in e th e d e p en d e n ce of a b so lu te d e te c to r efficiency on
g a m m a en erg ies a t d iffe re n t d ista n c e s from d e te cto r to source G a m m a sources are
p laced a t d ifferen t d is ta n c e s from detecto r surface In o u r e x p e rim e n t, g a m m a sources are placed a t po sitio n from 2 cm to 16 cm to su rfa c e of d etecto r
The a b so lu te efficiency of H P G e g a m m a G a m m a v is io n sp e c tro m e try are show n in fig.3
The a b so lu te efficiency of H P G e g a m m a G e n n ie 2000 s p e c tr o m e tr y a re show n
in fig.4
3 R e s u lt s a n d d i s c u s s i o n
F ittin g th e e x p e r im e n t d a t a for d e te rm in in g a b s o lu te efficiency of detector
w ith theorical fu n ctio n s is c a r r i e d out
In order to select th e m o st s u ita b le th eo rica l fu n c tio n for fittin g w ith
e x p e rim e n ta l d a ta T h e fittin g p a r a m e t e r s of e x p e r im e n ta l d a t a wich th eo rical functions:
a re com pared
In g en eral, th e fittin g d ia g ra m s of th e s e two fu n c tio n s a r e closing to
e x p e rim e n ta l points H ow ever, th e fu n ctio n (2) is m ore s u i t a b l e to h ig h en erg en tic
r a d i a t i o n s b e c a u s e B c o e f f i c i e n t s h a v e s m a l l e r f a i l u r e s , t h e r e f o r e , e r r o r a r e s m a l l
A c k n o w l e d g e m e n t s : T h e V ie tn a m N a tio n a l U n iv e rs ity , H a n o i su p p o r ts th is
w ork th ro u g h th e su b je ct QG-04-02
R e f e r e n c e s
1 Boston M., E r d u r a n M N ,S irin M a n d S u b a s t M., Isom eric cro ss-sectio n ra tio
for th e (n,2n) r e a c tio n on Sc from 13.6 to 14.9 MeV , Phys R e v , New York, V
56, No 2(1997), pp 918-921
2 w M a a h a r t a n d H Vonach, T h e g a m m a -ra y a b s o r p tio n coefficients for Nal(Tl),
Nucl.Instr Meth E lsev ier, V.134, No 4(1976), pp 347-351
3 Kolev D., S tu d ies of som e Isom eric Yield R atio s P ro d u c e d w ith B r e m s s tr a h lu n g ,
Appl R a d ia t i h o t , G r e a t B rita in , V.49, No 8(1998), p p 989-995.
4 Seuyng-Gy Ro, , A b so lu te dection efficienies of c y lin d ric a l N al(T l) c ry s ta l for
point source g a m m a -ra y s , J K o r A ss o R ad ia t Prot, K orea, V 8, No 3(2002), pp
235-241